Conventionally, inkjet printers that print an image by ejecting fine droplets of ink from a plurality of outlets of a head toward printing paper while moving the printing paper relative to the head have been used. Also, a pattern for density correction that indicates a constant density image has been printed on printing paper with a plurality of outlets and the distribution of densities in the image on the printing paper has been measured so as to acquire a plurality of density correction values of the outlets and achieve uniform print densities of the outlets using the density correction values (so-called shading compensation). For example, Japanese Patent Application Laid-Open No. 6-166247 discloses a method in which, in addition to a pattern A for use in detecting unevenness of the recording density of a recording head, a pattern B that is printed by driving only a specified recording element is formed simultaneously so that the positions of the recording elements and the detected density data are always properly associated with each other, which alleviates head shading.
Incidentally, with inkjet printers, ink ejection failures (also called “failing nozzles” or “missing dots”) may arise due to clogged outlets caused by ink hardening or adherence of debris such as paper dust or due to the effect of air bubbles in the ink inside the head. In this case, part of the printed image may be faint, or so-called “whiteout regions” on which no ink is deposited may appear. If an ink ejection failure occurs in any of a plurality of outlets when (at the time of or during) printing a pattern for density correction on printing paper, appropriate density correction values for the outlets cannot be acquired. In this case, it is necessary to perform cleaning processing and then re-print the pattern for density correction, thus taking a long time to acquire density correction values.